The present invention relates to a paper conveying device in a printing apparatus, such as a laser beam printer, for printing an image on cut paper.
In a conventional cut-paper laser beam printer, as shown in FIG. 1, paper feeding rollers 3 and conveying rollers 5 are rotated first in accordance with a printing command so as to send out an uppermost one of sheets of paper 2 received within a paper feeding cassette 1. When a paper detection device 6 detects the sent-out paper 2, the paper feeding rollers 3 are stopped. The conveying rollers 5 convey the paper 2 to registration rollers 7. Then, the paper 2 abuts the registration rollers 7 so as to be loosened as shown in the drawing so that the direction of meandering is corrected.
Next, when the front end of the paper 2 is made to reach a transfer corotron 9 by the rotation of the registration rollers 7, the registration rollers 7 are rotated in synchronization with the photosensitive drum 8 so that a toner image formed on the photosensitive drum 8 is transferred onto the paper 2 by corona discharge of the transfer corotron 9.
In the above-mentioned conventional paper conveying device of the printing apparatus, paper-feeding speeds of the respective rollers are made equal to each other in order to prevent looseness or extreme tension from occurring on the paper 2. Alternatively, the peripheral speed of the registration rollers 7 is made equal to that of the photosensitive drum 8 during image transfer in order to prevent displacement of the image from occurring while transferring the toner image. That is, the peripheral speed of the respective rollers is selected to be substantially equal to that of the photosensitive drum.
Further, where two sheets of paper are taken in piles from the paper feeding cassette 1, a separation roller 4 is stopped so as to stop the lower sheet of paper so that only the upper sheet of paper is conveyed and succeedingly the lower sheet of paper is sent out. In such a case, the lower sheet of paper is sent out upon detection of a rear end of the upper sheet of the paper by means of the paper detection device 6.
In such a conventional technique, if it is desired to make the speed of continuous printing high, it is necessary to shorten the interval between aligned sheets of paper, and to increase the paper conveying speed. However, to this end, it is necessary to increase the scanning or sweeping speed of a laser beam for exposing the photosensitive drum, and a corresponding increase in speed of all of the processes such as discharge of the photosensitive drum 8, development, cleaning, transferring, and so on. As a result, there have been difficulties in that the apparatus becomes large in size and the manufacturing cost becomes high correspondingly.
Further, in order to attain a high printing speed, the piled-sheet separation mechanism constituted by the separation roller 4, the conveying rollers 5, the paper detection device 6, etc. has been made small in size. However, as the mechanism is made small in size, the piled-sheets separating operation becomes unreliable.